The present invention relates to a semiconductor memory device, and more particularly, to a circuit for generating an internal voltage readily amenable to incorporation into a highly integrated semiconductor memory device.
As a semiconductor memory device is highly integrated, the elements thereof become more and more miniaturized, such as transistors which are constructive elements in a chip. In this case, when supplied with an external voltage that has been supplied to the elements not miniaturized, the miniaturized elements, such as the transistors, may be destroyed by the stress caused by the relatively strong electric field. Hence a semiconductor memory device that is highly integrated, that is, a device rated at sixteen megabytes (mega: 10.sup.6) or over in capacity, requires a circuit for generating an internal voltage by reducing the amplitude of an external voltage to the amplitude of the operating voltage for the chip. For example, a semiconductor memory device of sixteen megabytes in capacity must use an internal voltage of about four volts, and usually obtains that internal voltage by reducing the amplitude of an external voltage of five volts. Moreover, a semiconductor memory device of over sixteen megabytes of capacity will use external and internal voltages of even lower amplitude.
One of the more popular designs of a conventional internal voltage generator reduces the amplitude of an external voltage to a given level required for the operation of a semiconductor memory chip by applying an internal voltage provided by a driver stage to both the memory elements of the chip and to one input port of a comparator. If the internal voltage falls in amplitude, the reduced amplitude is detected by the comparator, which, in turn, lowers an output voltage. The driver is driven into greater conductivity in response to a reduction in the amplitude of the output voltage, thereby compensating for the drop in the amplitude of the internal voltage. The operational characteristics of comparators are well-known in the art, and a more detailed description of comparators is not necessary for this explanation, except to note that popular designs for internal voltage generating circuits typically produce a constant internal voltage despite receiving an external voltage which has a value exceeding a given level.
Such conventional internal voltage generating circuits however, can not provide an internal voltage equal to the external voltage in order to subject a semiconductor chip to a post-production quality test such as a "burn-in test" (i.e., a post-production test conducted by some of the more responsible semiconductor manufacturers in which finished semiconductor chips are subjected to a high test voltage that exceeds a given value at a high temperature for a long time so as to expose imperfect chips) because such internal voltage generating circuits are designed to always provide a particular internal voltage corresponding to normal operational modes of the chip in response to any externally applied voltage. Consequently, it is nearly impossible to readily check and identify the imperfect chips among batches of conventional internal voltage generators, thus resulting in a considerable loss of time during subsequent efforts to use unreliable chips as well as a reduction in the reliability of the semiconductor memory devices incorporating conventional internal voltage generators.